It is known to provide a cascade of a boost converter for PFC followed by a PWM (pulse width modulation) buck converter for producing a lower voltage than the typically high output voltage of the PFC converter, and to operate these in a synchronized manner using a single clock reference. Such cascaded converters are described for example in Hwang U.S. Pat. No. 5,565,761, issued Oct. 15, 1996 and entitled “Synchronous Switching Cascade Connected Off-Line PFC-PWM Combination Power Converter Controller”, and Hwang et al. U.S. Pat. No. 5,798,635, issued Aug. 25, 1998 and entitled “One Pin Error Amplifier And Switched Soft-Start For An Eight Pin PFC-PWM Combination Integrated Circuit Converter Controller”.
Another arrangement comprising cascaded PFC and PWM power converters is known from Fairchild Semiconductor Application Note 42047 entitled “Power Factor Correction (PFC) Basics”, Rev. 0.9.0, Aug. 19, 2004. Various PFC arrangements and their control are known for example from Chapter 1, entitled “Overview of Power Factor Correction Approaches”, of “Power Factor Correction (PFC) Handbook”, ON Semiconductor document HBD853/D, Rev. 2, August 2004, and from “The Dynamics of a PWM Boost Converter with Resistive Input” by S. Ben-Yaakov et al., IEEE Transactions on Industrial Electronics, Vol. 46, No. 3, June 1999, pp. 613-619, describing an indirect PFC converter control scheme.
It is desirable for the converter switching frequency to be relatively high, in order to reduce the sizes of reactive components. However, switching losses increase with increasing switching frequency, resulting in practical upper limits to the switching frequencies that can be used.
It is also known to reduce the PWM power converter switching losses by using a resonant mode power converter, taking advantage of zero voltage switching (ZVS) and/or zero current switching (ZCS). Examples of resonant mode converters include series resonant, parallel resonant, series parallel resonant or LCC, and LLC converters examples of which using a half bridge converter topology are described in Chapter 4, entitled “LLC Resonant Converter”, of “Topology Investigation for Front End DC/DC Power Conversion for Distributed Power System”, by Bo Yang in a dissertation submitted to the Faculty of the Virginia Polytechnic Institute and State University, Sep. 12, 2003. Among such resonant mode converters, an LLC converter is preferred for reasons explained in the dissertation.
An LLC power converter is also known for example from Blom et al. U.S. Pat. No. 6,437,994, issued Aug. 20, 2002 and entitled “LLC Converter Includes A Current Variation Detector For Correcting A Frequency Adjusting Control Signal Of An Included Difference Detector”.
An LLC converter has two resonant frequencies, namely a series resonant frequency and a parallel resonant frequency, and is typically designed to operate in a range between these resonant frequencies in which the gain of the circuit is negative, meaning that an increase in frequency decreases the energy transferred to the output of the converter. For example with a half bridge topology, the half bridge current lags the half bridge voltage due to a primarily inductive nature of the resonant tank in this range, so that the LLC can be operated to advantage with ZVS.
An LLC converter is thus operated with a variable frequency switching waveform, which is a substantially square waveform with dead times to avoid simultaneous conduction of the half bridge switches. A higher frequency corresponds to a lighter load. Although a particular LLC converter may be designed for operation over a relatively narrow range of frequencies, different LLC converters for use in different applications, and with potentially different input voltages, may be required to operate in very different frequency ranges over a wide frequency band.
STMicroelectronics Application Notes AN2321, “Reference design: high performance, L6599-based HB-LLC adapter with PFC for laptop computers”, August 2006 and AN2393, “Reference design: wide range 200W L6599-based HB LLC resonant converter for LCD TV & flat panels”, September 2006 disclose cascaded PFC and half bridge LLC power converters each using an L6563 controller for the PFC converter and a separate L6599 resonant controller for the LLC converter. Reference is also directed in these respects to STMicroelectronics data sheets L6563, “Advanced transition-mode PFC controller”, November 2006 and L6599, “High-voltage resonant controller”, July 2006.
It is also known, from Balakrishnan et al. U.S. Pat. No. 6,249,876, issued Jun. 19, 2001 and entitled “Frequency Jittering Control For Varying The Switching Frequency Of A Power Supply”, to reduce EMI (electromagnetic interference) emission by jittering the switching frequency of a switched mode power supply.
It is desirable to minimize the number of connections required for a control unit for an LLC converter, especially if the control unit is provided as an integrated circuit (IC) whether or not the IC also provides for control of a PFC converter. At the same time, it is desirable to provide for full control of the LLC converter, including for example determination of minimum and maximum switching frequencies, closed loop frequency control within the range of these frequencies, converter current sensing for overload protection, and input voltage monitoring for soft start of the LLC power converter.
In addition, it is necessary to maintain an accurate matching of the on-times of the switches of an LLC converter, over all of its potentially very large range of possible switching frequencies. While these on-times Ideally would be exactly 50% of the period at any switching frequency, in practice, as is well known, it is necessary to provide dead times which reduce the on-times to slightly below 50% to avoid simultaneous conduction of the switches at the switching times. Accordingly, it is desirable for the dead times also to be closely matched. Furthermore, it is desirable that the dead times be minimized for any given switching frequency; this presents a problem in view of the wide range of possible switching frequencies of the LLC converter.